Automotive heat exchangers that use a pumped, liquid heat exchange medium, as opposed to a compressed gaseous/liquid heat exchange medium, include radiators and heaters. Typically, these include two elongated manifolds or header tanks, one on each side of the heat exchanger, with a central core consisting of a plurality of evenly spaced, flattened flow tubes and interleaved corrugated air fins running between the two tanks. Each tank is generally box shaped, with parallel side walls, a back wall joining the side walls, two axially opposed ends, and an open area opposite the back wall, which is eventually closed off when it is fixed leak tight to one side of the core. Each header tank distributes pumped liquid to or from the flow tubes in the core, and is in turn filled or drained by an inlet or outlet pipe opening into the header tank at a discrete location. In typical modern radiators, the header tank is a molded plastic box, and the inlet or outlet pipe is integrally molded to one of the side walls. The pipe, therefore, is oriented both perpendicularly to the length of the tank and perpendicular the flow tubes. Coolant flow entering the inlet pipe must, therefore, turn ninety degrees toward the two ends of the tank before as well as turning ninety degrees again to flow out of the tank interior and into the flow tubes. The converse is true for coolant exiting the return tank through the outlet pipe. An example of a recent radiator with molded plastic, box shaped header tanks may be seen in U.S. Pat. No. 5,762,130, which is fairly typical in its basic flow configuration, apart from being a U flow design, with the inlet and outlet pipe located on one tank. The orientation of the pipes relative to the tank walls and flow tubes is as described above, however.
The design of a radiator or any cross flow heat exchanger with a liquid medium flowing in one direction through flow tubes, and with air blown perpendicularly across the flow tubes, is a compromise between heat exchange efficiency between the two flowing media, and the pressure or pumping losses of the two media. For example, it is well known that decreasing the flow passage cross sectional area will present relatively more surface area of the fluid medium within the flow passage to the air blowing over the flow tube, increasing the heat transfer efficiency from fluid to air. A tube that is smaller on the inside is also thinner on the outside, and so presents less obstruction the air blown over the outside of it, decreasing the air side pressure loss through the core. However, a thinner flow tube creates more fluid pressure loss through the tube, end to end. Some compromise can generally be found between air side pressure drop, tube thickness, and liquid (coolant) pressure drop. However, the ability to reduce total coolant pressure loss (pumping loss) elsewhere in the heat exchanger would allow the use of thinner tubes in general, which would be very positive, considering that thinner tubes also decrease air side pressure loss.
One source of coolant pressure drop through the heat exchanger that has not received a great deal of attention in the prior art is turbulence or "turning" losses that occur at the transition between the pipe opening and the enclosed interior of the header tank, especially the inlet pipe. That is, since the inlet pipe typically enters through a tank side wall, and not the tank back wall, it is oriented perpendicular to the flow tubes, as well, and must change direction both to reach the opposite ends of the tank and in order to flow into the tubes. The turning transition is not a great source of pressure loss when the interior volume of the tanks is large, since a large interior volume can act as a large pressure reservoir to "absorb" and distribute coolant to the flow tubes. As available underhood space shrinks, however, radiator header tanks become smaller, and the parallel side walls become closer. Flow exiting the opening of the inlet pipe (through the first side wall) impinges on the proximate, opposed second side wall, creating turbulence and pressure loss before it can be distributed toward the opposite ends of the tank and into the flow tubes.
The other liquid medium heat exchanger typically found in an automobile, the heater core, has a similar cross flow configuration, but faces a different problem. There, the inlet pipe generally opens through the back wall of the header tank, in line with, rather than perpendicular to, the flow tubes. The flow thus impinges directly onto the ends of the nearest aligned flow tubes, rather than against a side wall of the tank, which would theoretically be positive, in terms of direct flow into the tubes with minimal pressure loss. However, the fact that the ends of the nearest tubes are in line with the inlet pipe is a detriment, because the force of the impinging flow against the near tube ends erodes and damages them. Therefore, it has been proposed in several heater core designs to place a protective tent or baffle like structure between the inlet pipe opening and the ends of the nearest aligned flow tubes. These act as a road block, in effect, interrupting the flow at that point, rather than smoothing it out, and would actually increase total coolant pressure drop across the core. This is an acceptable price in that context, however, since it is considered necessary to protect the otherwise eroded tubes.